I don't think you have to say much.
I would try an arrange adults only outings, or, go on an outing but leave your DS at home. Try and arrange coffee, etc, when someone can watch your kid. Phones and emails are really great ways to stay in contact (although I know they are not the same).
If she does want to do playdates, I would just be honest, but non-accusatory. "Ds and your Ds fight too much - we have decided to skip playdates for now."
Unless she is in complete denial, she knows her son is overly aggressive. You don't have to have a heart-to-heart with her, you just have to keep your child safe.
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